The invention relates to a hydraulic driving device for a vehicle, having a pressure source generating a predetermined pressure for maintaining the operating pressure of a hydraulic motor, a braking cylinder with a spring chamber comprising a spring arrangement for operating a braking arrangement and a release chamber that can be acted upon by pressurized fluid from the pressure source against the force of the spring arrangement with the purpose of retaining the braking device in its release position, and having a braking control valve that, in a release position, releases a flow from the pressure source to the release chamber
This invention relates to a driving device particularly suited for agricultural and industrial working vehicles, like, for instance, tractors, harvesters or construction machines, and is mainly used in compact loaders.
A driving device related to this invention is shown in EP 0 909 690 A2. Further to a pump supplying, in a closed circuit, a hydraulic motor, which drives wheels of the vehicle, it comprises an auxiliary pump, which generates a predetermined load pressure, which is also supplied to the release chamber in the braking cylinder. When the load pressure ceases, only the spring arrangement acts to brake the vehicle. When, however, the braking control valve is activated, the load pressure in the release chamber renders the spring arrangement in the spring chamber inactive. When, however, a fault occurs, which requires braking; the braking control valve is deactivated, so that the spring arrangement, supported by the load pressure in the spring chamber, ensures a safe braking, also in relation to a rotation of the hydraulic motor. On swift movements of a tool operated by the working hydraulics, however, cases may occur, in which the pressure supplied to the braking cylinder gets too low, so that the braking device is activated accidentally, thus braking the vehicle. This may, for example, be the case, when traction acts upon the piston rod of a working cylinder, for example when a grab of an excavator, filled with earth, is lowered. On a swift movement of piston and piston rod, more fluid then flows into than out of the working cylinder, as the active surface of the side of the piston, which is connected with the piston rod, is reduced in relation to that of the other end of the piston by the cross-sectional area of the piston rod. At the same time, an increased fluid consumption occurs. Further, the pressure in the release chamber of the braking cylinder may briefly drop below the release pressure, when the speed of the combustion engine driving the pump, and through this also the hydraulic motor and the driving wheels, is reduced by the driver or by an increasing load on the vehicle during operation.
Also in connection with the driving device disclosed in the German patent application 199 62 807, not yet published, similar cases may occur, as described in detail below on the basis of FIG. 1 of the enclosed drawings.
Therefore, a principal object of this invention is to provide a hydraulic driving arrangement for a vehicle in which an unintended activation of the braking device, without the occurrence of an emergency, is prevented.
These and other objects will be apparent to those skilled in the art.